Optical pickup head wherein an error signal is produced by detecting a phase difference in diffraction beams from a holographic optical element

摘要

A light source emits a light beam which is focused on an optical recording medium. The light beam is reflected and diffracted by the optical recording medium. A holographic optical element has divided regions formed with different holographic patterns. The divided regions of the holographic optical element diffract the light beam reflected and diffracted by the optical recording medium, and thereby generate diffraction light beams respectively. The divided regions of the holographic optical element lie in quadrants of an imaginary X-Y coordinate system respectively. An X axis of the coordinate system extends in a direction corresponding to a direction of a pit sequence on the optical recording medium, that is, a tangential direction of the optical recording medium. A Y axis of the coordinate system extends in a direction corresponding to a direction perpendicular to the direction of the pit sequence, that is, a radial direction of the optical recording medium. Photodetectors receive the diffraction light beams generated by the divided regions of the holographic optical element, and convert the received diffraction light beams into corresponding electric signals. Phases of the electric signals are compared, and a tracking error signal is generated on the basis of a result of the phase comparison.